[Fix] Fix bug in 1-benchmark_valid.py if checkpoint_root is None.

[mzr1996]

Motivation

In 1-benchmark_valid.py, if args.checkpoint_root is None, it will raise an error.

Modification

Fix the bug.

BC-breaking (Optional)

No

Use cases (Optional)

No

Checklist

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, like docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with those projects, like MMDet or MMSeg.
• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

Merging #489 (57068c2) into master (63a2211) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #489   +/-   ##
=======================================
  Coverage   77.96%   77.96%           
=======================================
  Files         102      102           
  Lines        5619     5619           
  Branches      923      923           
=======================================
  Hits         4381     4381           
  Misses       1111     1111           
  Partials      127      127           

Flag	Coverage Δ
unittests	77.96% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmcls/apis/inference.py	19.64% <0.00%> (ø)

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[Ezra-Yu]

LGTM.